Signaling systems have been used in the past to signal and locate the occurrence of emergency situations such as crimes, fires, or accidents, and general tracking such as package and freight shipping or road-side assistance. Prior art signaling and locating systems have used signal transmitting devices or units and signal receiving units located at various locations within a very limited geographic area to provide for such signaling and location determination within a limited area.
Prior art systems, such as the Infrared Personal Locator System described in U.S. Pat. No. 4,275,385, require the use of an entirely new system to receive and process the signals as well as transmitting devices for sending the signals. Other existing locating systems include cellular handsets outfitted with Global Positioning System receivers.
A recent innovation is the ability to locate Cellular telephones as described in U.S. Pat. No. 5,327,144. The cellular telephone location system is an overlay to an existing cellular system, such as the Advanced Mobile Phone System (AMPS system), which is capable of locating all existing standard cell phones in North America without the need to modify the cellular system in any fashion.
The Federal Communications Commission (FCC) has allocated a 25 MHz frequency spectrum for use by such cellular systems. The spectrum is divided into two 12.5 MHz bands, known generally as A and B bands, having both control channels and voice channels. The cellular telephone system typically includes multiple cell sites (or base stations) and a centrally-located cellular switch, called a Mobile Telephone Switching Office (MTSO).
Each cell site typically uses only a single control channel. The control channel from a cell site to a mobile unit, such as a cellular telephone, is called the "forward" control channel and the control channel from the cellular telephone to the cell site is called the "reverse" control channel (RCC). The cellular telephone system uses the RCC signals for a variety of reasons, the most common of which are: registration bursts that indicate to a local cell tower (or cell site) that the phone is within its coverage region--typically to effect the proper routing of incoming calls; page responses which are returned to the cell system in response to a page request by the cell system; and origination requests which are used to initiate the set up of a normal phone call.
The prior art cellular telephone location system such as disclosed in U.S. Pat. No. 5,327,144 locates cellular telephones by placing multiple receivers (e.g. AMPS receivers)--typically located at existing cell sites, which detect and digitally sample all RCC transmissions or bursts. These samples are forwarded to a central site and are correlated to determine the time difference of arrival (TDOA) of the signal at the various cell sites from which a location is thereby computed.
Information embedded in current RCC signal transmissions or bursts includes, but is not limited to: the Mobile ID (MID), e.g. the "cell-phone number"; the Electronic Serial Number (ESN) that identifies the specific handset; the number being called (if an origination request); and the system ID. Whether the cellular telephone transmitting an RCC burst is located depends on the information transmitted. For example, the information in the RCC burst is used at the central site to filter specific RCC bursts for processing--typically reserved for emergency situations and for subscribers paying for location service.
Such a location system is limited to cellular telephone location but is not designed for use by a non-subscribing individual to signal an emergency or other situation or for use in tracking. Also, the signals used in such a cellular system are strictly regulated by the FCC and must comply with the frequency spectrum allocated and signal definition, which limits the number of cellular users and usage of the cellular system for emergency signaling and tracking.